Abstract. The mountain cryosphere of mainland Europe is recognized to have important impacts on a range of environmental processes. In this paper, we provide an overview on the current knowledge on snow, glacier, and permafrost processes, as well as their past, current, and future evolution. We additionally provide an assessment of current cryosphere research in Europe and point to the different domains requiring further research. Emphasis is given to our understanding of climate–cryosphere interactions, cryosphere controls on physical and biological mountain systems, and related impacts. By the end of the century, Europe's mountain cryosphere will have changed to an extent that will impact the landscape, the hydrological regimes, the water resources, and the infrastructure. The impacts will not remain confined to the mountain area but also affect the downstream lowlands, entailing a wide range of socioeconomical consequences. European mountains will have a completely different visual appearance, in which low- and mid-range-altitude glaciers will have disappeared and even large valley glaciers will have experienced significant retreat and mass loss. Due to increased air temperatures and related shifts from solid to liquid precipitation, seasonal snow lines will be found at much higher altitudes, and the snow season will be much shorter than today. These changes in snow and ice melt will cause a shift in the timing of discharge maxima, as well as a transition of runoff regimes from glacial to nival and from nival to pluvial. This will entail significant impacts on the seasonality of high-altitude water availability, with consequences for water storage and management in reservoirs for drinking water, irrigation, and hydropower production. Whereas an upward shift of the tree line and expansion of vegetation can be expected into current periglacial areas, the disappearance of permafrost at lower altitudes and its warming at higher elevations will likely result in mass movements and process chains beyond historical experience. Future cryospheric research has the responsibility not only to foster awareness of these expected changes and to develop targeted strategies to precisely quantify their magnitude and rate of occurrence but also to help in the development of approaches to adapt to these changes and to mitigate their consequences. Major joint efforts are required in the domain of cryospheric monitoring, which will require coordination in terms of data availability and quality. In particular, we recognize the quantification of high-altitude precipitation as a key source of uncertainty in projections of future changes. Improvements in numerical modeling and a better understanding of process chains affecting high-altitude mass movements are the two further fields that – in our view – future cryospheric research should focus on.

International audienceUsing molecular dynamic simulations, we investigate the characteristics of dense flows of model cohesive grains. We describe their rheological behavior and its origin at the scale of the grains and of their organization. Homogeneous plane shear flows give access to the constitutive law of cohesive grains which can be expressed by a simple friction law similar to the case of cohesionless grains, but intergranular cohesive forces strongly enhance the resistance to the shear. Then we show the consequence on flows down a slope: a plugged region develops at the free surface where the cohesion intensity is the strongest. Moreover, we measure various indicators of the microstructure within flows which evidence the aggregation of grains due to cohesion and we analyze the properties of the contact network (force distributions and anisotropy). This provides new insights into the interplay between the local contact law, the microstructure and the macroscopic behavior of cohesive grains

Using discrete numerical simulations, we have studied the flow down a rough inclined plane of a bidisperse assembly of frictional cohesionless disks. Our study focuses on steady uniform flows, once a stable segregation has developed inside the flowing layer. The material is segregated in three layers: a basal layer (small grains), a superficial layer (large grains), and a mixed layer in the center, so that the average diameter of the grains increases from the bottom to the top. From the measurement of the profiles of velocity, solid fraction, and stress components, we show that the rheological law of such a polydisperse material may be described by a local friction law, which extends the result obtained for quasimonodisperse granular flows. This law states that the effective friction coefficient depends approximately linearly on a generalized inertial number, taking into account the average diameter of the grains.

We investigate the rheology of dense cohesive granular media outside the quasistatic regime using molecular dynamic simulations of homogeneous plane shear flow in the steady state. Friction as well as dilatancy are governed by two dimensionless numbers, the inertial number and the cohesion number. The internal friction coefficient and the solid fraction depend linearly on the inertial number, as in the cohesionless case. Weak cohesion leads to an expansion of the material, while the friction remains unchanged. Above a characteristic cohesion strength, a strong increase of friction combined with a decrease of solid fraction is observed. We relate this behavior to the growing space-time heterogeneities and to the increase of the cohesion-enhanced microscopic friction between the grains.

ABSTRACT. The evaluation of avalanche release depths constitutes a great challenge for risk assessment in mountainous areas. This study focuses on slab avalanches, which generally result from the rupture of a weak layer underlying a cohesive slab. We use the finite-element code Cast3M to build a mechanical model of the slab/weak-layer system, taking into account two key ingredients for the description of avalanche release: weak-layer heterogeneity and stress redistribution via slab elasticity. The system is loaded by increasing the slope angle until rupture. We first examine the cases of one single and two interacting weak spots in the weak layer, in order to validate the model. We then study the case of heterogeneous weak layers represented through Gaussian distributions of the cohesion with a spherical spatial covariance. Several simulations for different realizations of weak-layer heterogeneity are carried out and the influence of slab depth and heterogeneity correlation length on avalanche release angle distributions is analyzed. We show, in particular, a heterogeneity smoothing effect caused by slab elasticity. Finally, this mechanically based probabilistic model is coupled with extreme snowfall distributions. A sensitivity analysis of the predicted distributions enables us to determine the values of mechanical parameters that provide the best fit to field data.